An atomic force microscope cantilever has been used as a bending-beam sensor to measure surface stress changes which occur during electrochemical processes. The mechanical properties of the lever and the sensitivity of the detection system mean that the sensor is both fast and sensitive. Surface stress changes presented for the electrochemical deposition and stripping of a Pb monolayer on an Au(111) surface show features which match peaks in the cyclic voltammogram and can be understood by reference to the known surface structure determined by STM, AFM, and grazing incidence XRD. There is a pronounced reduction in the stress derivative at the potential corresponding to the rotational phase transition of the lead monolayer. In the electrocompression region which follows monolayer form ati on, there is an essentially linear increase in compressive stress which may be modeled lo within 50% accuracy by a simple linear elastic model.